link: prevent deadlock when prelink tasks fail

If an error occured which prevented a prelink task from being queued,
then `pending_prelink_tasks` would never be decremented, which could
cause deadlocks in some cases. So, instead of calculating ahead of time
the number of prelink tasks to expect, we use a simpler strategy which
is much like a wait group: we add 1 to a value when we spawn a worker,
and in the worker function, `defer` decrementing the value. The initial
value is 1, and there's a decrement after all of the workers are
spawned, so once it hits 0, prelink is done (be it with a failure or a
success).

Author: mlugg